1. Field of the Invention
This invention relates to power supplies and in particular to a power supply apparatus and method for driving circuits having constant current devices.
2. Description of the Related Art
Advances in light emitting diode (“LED”) technologies have resulted in devices that are brighter, more efficient and more reliable. LEDs are now being used in many different applications that were previously the realm of incandescent bulbs; some of these include displays, automobile taillights and traffic signals. As the efficiency of LEDs improves, it is expected that they will be used in most lighting applications.
Channel letters are commonly found on the outside of buildings and are often used to advertise the name of the business. They are typically constructed of an aluminum or plastic housing having the shape of a letter and is approximately 5″ deep. The housing has a generally U-shaped cross-section, with the top opening in the housing covered by a colored translucent lens that transmits light from within the housing.
Channel letters are typically illuminated with neon or fluorescent light sources that are mounted within the channel letter housing. Neon and fluorescent lights provide a bright and continuous light source that allows the channel letters to be visible at night. These light sources, however, have a relatively short life (20,000 hours), are fragile, operate at high voltage (7,000 to 15,000 volts for neon) and can consume a relatively large amount of power. Neon bulbs can also experience difficulty with cold starting, which can lead to the bulb's failure.
LEDs are more frequently being used as the light source in many different applications. U.S. Pat. No. 5,697,175, to Schwartz, discloses a low power illuminated sign that is particularly adapted for use with common EXIT signs over doorways. The back of each sign comprises a reflector with a series of cavities with curved surfaces. Each cavity corresponds to a letter and background area in the sign. LEDs are mounted in the center of the cavities to illuminate the letters or background area. The LEDs are provided on a separate perpendicular circuit board or on a central projection formed in the bottom of the cavities, with light from the LEDS directed outward. The letters and background area of the sign are illuminated by light reflecting forward from the curved surfaces of the cavities, so that the only visible light is from the illumination of the cavities.
U.S. Pat. No. 6,042,248, to Hannah et al., discloses an LED assembly for channel letter illuminating signs having an enclosure/housing covered by a translucent lens. Each sign includes a plurality of track moldings at the base of its enclosure, with the moldings running along the longitudinal axis of the sections of the channel letter. Linear arrays of LEDs are mounted on printed circuit boards (PCBs) that are then mounted in the track moldings. Each track molding can hold two PCBs in parallel with each of the PCBs arranged on a longitudinal edge, with the LEDs directed outward.
LED based channel letter lighting is also available from LumiLEDs, Inc., under part numbers HLCR-KR-R0100 and HLCR-KR99-R0200, which comprises LEDs that are each mounted by insulation displacement connectors (IDC) on two inch centers. The chain of LED modules is then mounted into a bendable clip or rail, each of which are then mounted inside a channel letter to hold the LEDs in place. Power is provided by a combination of an AC/DC mother power supply and a DC/DC daughter power supply. A sensing LED is also included as a temperature and current sensor.